How to tell it's time to replace your subfloor

A good subfloor is key to getting the best performance out of your new flooring. Before you can even think about laying down your new flooring, you need to check the existing subfloor is in good condition, and this is the best time to check, when your flooring is removed. If it isn't, any imperfections underneath, like lumps, dips, and wobbly bits, will be reflected in any flooring laid on top!

In this helpful guide, you’ll learn about the different types of subfloors, how to spot a good subfloor from a bad one, and how to know it’s time to improve your subfloor, as well as how to fix minor issues to prepare your subfloor for your new flooring, with a little help from professional flooring experts like us, of course.

In this guide:

What is a subfloor?
What are the different types of subfloor?
What does a good subfloor look like?
4 signs you need to replace your subfloor
How to replace a subfloor
What happens if my subfloor needs work?

What is a subfloor?

In a nutshell, a subfloor is the surface on which a floor covering is laid. Every floor is made up of different layers: the joists (the structural element which supports everything on top), the subfloor, the underlay (depending on the flooring type), and finally, the decorative floorcovering on top, like carpet, vinyl, or laminate.

What are the different types of subfloors?

The three main types of subfloor are:

wooden floorboards
screed
concrete

All of these have their different benefits. Which type of subfloor you have in your home will depend on many factors, such as how old your house is. You might find you have different subfloors in different rooms if your home has been extended or renovated over the years.

What does a good subfloor look like?

Before the flooring can be laid, you need to make sure your subfloor is dry, smooth, level, structurally sound, and free from debris. A subfloor in poor condition will need to be repaired before new flooring can be installed - this should always be done by a professional. Here are four signs of a good subfloor:

1. Dry

This means completely free from moisture, not just dry to the touch. This is especially important if you've recently had a concrete or screed subfloor laid, which often takes a minimum of one day's drying time per millimetre of thickness. To test for moisture in your concrete subfloor, tape a square of polythene to it and leave it overnight. If there's condensation on the underside of the polythene, or the concrete underneath is a different colour to the rest of the subfloor, then it's not fully dry yet.

2. Level

To find out if your subfloor is uneven, place a 2m straight edge or spirit level on the floor. The tolerance on the undulations is 2mm, which is roughly the thickness of a £1 coin. If you can slide the £1 coin under the straight edge or spirit level, then your subfloor will require some correction by professionals before your new flooring can be laid. Sometimes all that’s needed is a self-levelling compound over the top of your existing subfloor to smooth it out (but this will need to dry before a floorcovering can be laid).

3. Structurally sound

The subfloor should be solid, stable, and able to provide support for anything placed on top. If you have a wooden subfloor, all the floorboards will need to be nailed or screwed down securely or covered with plyboard if there are too many big gaps, before any new flooring can be laid.

4. Free from debris

There should be no loose debris on the surface of the subfloor such as dirt, dust, disintegrated underlay, or old adhesive. Any big lumps of glue need to be scraped off and the subfloor should have a thorough sweep and a vacuum before the new flooring goes down (which all good fitters will do).

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4 signs you need to replace your subfloors

1. Musty smell

If a musty or damp smell is coming up from your floors and you can’t detect any damp or mould inside the room, then chances are your subfloors may be compromised with damp. This can be structurally unsafe, as your subfloors may be damaged to the point that they’re no longer providing enough support, so this is really important to get investigated. Whilst it's unlikely, it might be that the mould is coming from your carpet.

2. Leaking ceiling

If you have a leaking ceiling, then chances are that something has been compromised on the floor above and this has come through your floor coverings and subfloor. As plywood and concrete typically absorb moisture, by the time water has gotten through the ceiling, the materials may be completely saturated, so in addition to a plumber, you’ll want to call a flooring expert to make sure your subfloors on that floor aren’t damaged beyond repair.

3. Floor covering is damaged or separating

If your laminate floor is separating, cracking or bowing, or your vinyl floor is bubbling, these are all signs there could be major problems with your subfloors. Your subfloors should be solid and dry, and if it becomes weak or wet, these problems can occur, so it’s best to call up one of our flooring experts to take a look. We’ve got plenty more signs you can look out for outlined in our care guides, and some quick fixes you can make in our DIY corner.

4. Squeaky floors

Squeaky floors don’t always automatically mean there’s a problem with your subfloors, but they can be an indication that something isn’t right below your floor covering. This tends to happen with laminate and engineered wood more than vinyl and carpet, but if you have plywood subfloors and these types of flooring are making unusual noises, then you should get this checked.

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How to replace a subfloor

It can be hard to tell if your subfloor needs repairing or replacing, but if you order your new flooring from Tapi, we can assess your subfloor for you during a home visit or when we do our measurement checks. Don't worry, we won't be ripping up your existing flooring to look underneath! Phew! Instead, our surveyor will walk around to listen out for loud squeaky floorboards and to feel if the subfloor moves about or shifts. They'll also ask questions about when your house was built and whether you've recently had an extension to assess the condition and type of subfloor that's underneath. If they're still unsure, they'll ask a fitter to pop round and double check prior to fitting.

Another thing they’ll bear in mind is the type of floorcovering being laid on top, as that can make a difference to the amount of work that needs doing to the subfloor. Because of its thickness and flexibility, carpet can disguise small imperfections in a subfloor, whereas the same can’t be said for the more rigid laminate or luxury vinyl planks. But don’t let that influence your choice of flooring! A less than perfect subfloor can be sorted out very easily by professional fitters.

What happens if my subfloor needs work?

We'll always aim to keep any additional works to a minimum, but should we discover your subfloor needs repairing, we can provide you with a quote. If you're happy to go ahead, we'll arrange for independent fitters to carry out these works prior to installing your new flooring. We want your new floor to be the best it can possibly be - and that starts and ends with the subfloor. Get in touch with our experts here at Tapi by popping into your local Tapi store, or booking a free home visit for our floorologists to come to you to sort all your subfloor problems today.

Alternatively, for more flooring inspiration, discovr our Ideas Hub, where you can find out everything from the best underlay for vinyl flooring, through to the best flooring for underfloor heating.